>>11415190This is a deep mystery, nobody knows for sure. It is possibly related to establishing an elastic wave profile along the bottom which is in a dynamic steady-state with the jostling of the contact points, but you need good measurements to make a theory, and good measurements are difficult because of the issue of solid-solid sticking. The research field which studies questions of this sort is moderately active still, and was very active twenty years ago. It is called depinning. In depinning, there are models which partially explain this effect, although whether these models are relevant for a real solid-on-solid interface remains to be seen.
